Add support for Job Pools

[dmoody256]

Several other build systems use this idea, which is to group tasks together so that a given group will only utilize a certain number of cpu threads/cores out of the total number of threads/cores originally given to the build. This can improve overall build speed by preventing long jobs from bottle-necking dependencies, or organize jobs which don't run in parallel well (like high IO) from running together.

Looking at the Jobs class, I think that class will map pretty well to this concept.

Required information

• Link to SCons Users thread discussing your issue.
  https://pairlist4.pair.net/pipermail/scons-users/2020-May/008166.html
  https://pairlist4.pair.net/pipermail/scons-users/2016-January/004521.html